The molecular mechanism of polar arrest of replication forks, replicative helicases and RNA chain elongation by the bacterial replication terminator proteins will be investigated using x-ray crystallography, mutagenesis and biochemical analysis of the mutant forms of the terminator proteins. The experiments should provide definitive evidence as to whether not only DNA protein interaction between the terminus DNA and the terminator protein but also protein-protein contacts between the arresting and the arrested protein contributed to replication termination. The physiological role of a terminator protein as checkpoints of replication will be investigated by molecular cloning and in vivo analysis. A genetic selection scheme will be used to attempt to clone and study genes from eukaryotic cells that encode replication fork-arresting protein. X-ray crystallography will be use to determine the structure of a terminus-terminator protein complex.